Ross Johnston was born on February 18, 1994, in Charlottetown, Prince Edward Island. He played four seasons in the Quebec Major Junior League before signing as a free agent with the New York Islanders on March 31, 2015.

Johnston would spend the majority his first full professional season with the Bridgeport Sound Tigers of the American Hockey League. In 39 games for the club the big winger scored a goal and added three assists. On April 10, 2016, the last day of the regular season, Johnston made his NHL debut, suiting up for the Islanders in a 5-2 loss against the Philadelphia Flyers. Johnston would see action in 24 games for the Islanders during the 2017-18 season, scoring three goals and adding three assists.